This invention relates to a filament structure, and, more particularly, to a filament structure which is suitable for use in incandescent lamps.
Filaments for incandescent lamps generally comprise a filament with a coiled portion and an uncoiled portion where the coiled portion is the actual glowing part of the filament. The uncoiled portion of the filament is used for attaching the complete filament structure to a lead wire. For this purpose, it is known to place a tube on the free end of the uncoiled portion, and the tube is in turn welded to the lead wire. This procedure is necessary because the tungsten filament cannot be welded easily to the lead wire which is usually made of molybdenum. It is preferable if the tube is also made of molybdenum.
U. S. Pat. No. 5,808,399 discloses a method for fixing a filament onto a lead wire in an incandescent lamp. A molybdenum foil is used in this known method. The foil is wrapped around the free ends of the uncoiled portions of the filament and then squeezed together at its ends. The fin-like ends of the foil may come in the way of the welding electrodes which leads to inferior welding.
Instead of wrapped foils, it is also known to use tubes, which are pulled on the filament ends. Usually, the molybdenum tube is made of a strip material which is rolled into a tube. As a result, there is an axial slit in the mantle of the tube. The existence of the slit, and, more precisely, its positioning during the manufacturing process of the filament structure is a cause of certain problems. In the manufacturing process, the tube is pressed onto the uncoiled portion of the filament. If the tube comes between the pressing jaws in an unfavorable position, the slit may open too wide. If this wide slit rolls between the electrodes during the welding process, a substandard electrical contact arises between the filament and the tube since the welding machine is adjusted to weld (and melt) a pre-determined quantity of material. When a wide slit comes in the way of welding, the quantity of material to be welded will be smaller which leads to poor electrical contact.
Thus there is a particular need for a filament structure which alleviates the above negative effects and provides for positioning of the tube in order to accomplish a standard quality of welding.
In an exemplary embodiment of the invention, a filament structure comprises a filament with a coiled portion and an uncoiled portion. The uncoiled portion of the filament is provided with a tube disposed on a free end of the uncoiled portion. The tube has a central axis and two ends. At least one end of the tube comprises an extension which projects positively above a medium plane of the end of the tube. This medium plane is perpendicular to the central axis of the tube. By the term xe2x80x9cmedium planexe2x80x9d, it is indicated that this plane is approximately at an equal distance from the end points of the end of the tube.
A method for positioning a tube on the free end of a filament is also disclosed. In an exemplary performance of the method, the following steps are comprised.
The free end of a filament is inserted in a slit bet ween two parallel support surfaces so that the free end is projecting upward. The width of the slit is positively smaller than the diameter of the tube. There is provided an extension on an end of the tube, and the extension projects positively above a medium plane of the end of the tube. Said medium plane is perpendicular to a central axis of the tube. The tube is placed on the free end of the filament so that the free end of the filament is inserted in the tube substantially along the total length of the tube. In this position, the free end of the filament is substantially concentric with the central axis of the tube, and the extension of the tube projects downward towards the support surfaces. Due to the force of gravity, the lower end of the tube abuts at least one of the support surfaces. The filament is advanced in the slit along the support surfaces. During the movement of the filament, the tube is rotated around the free end of the filament by frictional forces arising between the extension of the tube and one of the support surfaces. The frictional forces arise upon the relative movement between the extension and the corresponding support surface.
This filament structure and method provide the advantage that the tube always arrives at the welding station in a well-defined position. Due to the substantially identical position of the tube, the slit of the tube can always avoid the way of welding. This results in a reliable and standard quality weld. A further advantage is that the proposed method does not require the use of sophisticated additional equipment, and may be readily applied to existing manufacturing facilities.